1. Field of the Invention
This invention relates to valve actuators; to valve actuators for valves on gas and product pipelines; to such actuators with two pistons; to such actuators with a fixed piston and free-floating piston; and to such actuators whose use does not result in the discharge of gas or liquid to the environment.
2. Description of Related Art
The prior art discloses a variety of valve actuators, valve actuators for valves on gas pipelines, and hydraulic operating systems for pipeline valves. Certain prior art systems use a closed hydraulic circuit's pressure applied to a piston which operates against a spring. In the event of a pipeline leak or break, pressure against the spring is released and the spring closes a valve in the pipeline until the emergency situation can be corrected. Then the system is re-pressurized to open the valve.
One problem with such systems is that in their natural environment they are exposed to a wide range of ambient temperature fluctuations. In colder situations after the system is initially set, contraction of hydraulic fluid and the associated reduction in pressure on the operating spring can cause an undesirable valve drifting situation. As the ambient temperature increases, the expansion of fluid in the closed system can apply undue pressure on system components which can result in failure. To solve this problem, one prior art system has a relief valve so that when warmer conditions exist, expanded hydraulic fluid blows by the relief valve to prevent undue pressure from being placed on the operating spring.
In another prior art system a hydraulic accumulator is provided to receive excess hydraulic fluid upon thermal expansion. Such accumulators increase the expense of the system and present another problem encountered by the system with the relief valve; i.e., when the temperature decreases, hydraulic fluid in the accumulator cannot return to the actuator.
Another prior art system which does not exhaust fluid to the environment has a tank which holds a quantity of unpressurized hydraulic fluid and a pump that provides the fluid under pressure to a valve actuator. The valve actuator has an actuator cylinder with a piston which is operatively connected to the pipeline valve so that fluid under pressure moves the piston compressing a spring which opens the valve. When the piston ceases movement, continued pumping moves another piston of a smaller diameter than the first piston, which also acts against the spring. An indicator rod gives a visual indication of the position of the first piston and when it is at approximately its midstroke location, pumping of the fluid is terminated and the actuator is set. When ambient temperature conditions change, the second piston moves to accommodate such changes thereby leaving the first piston unaffected. When a pressure sensing valve senses a predetermined pressure change in the fluid in the pipeline, a dump valve is opened permitting the pressurized fluid in the system to be transferred back to the tank. The spring moves the first piston to close the pipeline valve. Although there is not intentional exhaust from this system to the environment, pressurized fluid in it can slowly leak down allowing the spring to close the pipeline valve.
Particularly in locations remote from pipeline compressor stations, valve actuators for pipeline valves are used which are powered by the pipeline fluid (gas or liquid). In normal circumstances this is a dependable source of power for such actuators. Other power sources, such as stored nitrogen, hydraulic fluid pressure or electrical power, are not considered to be as dependable a power source in an emergency situation, such as a break in the pipeline.
Certain prior art valve actuators use the pipeline natural gas to directly power a cylinder or a gas-over-oil system in which hydraulic fluid is pressurized by the gas. In such situations once the valve actuator has been positioned, the natural gas used to power the actuator is discharged through a relief valve, exhaust valve or control valve into the atmosphere where it may harm the environment.
There has long been a need for an efficient and effective valve actuator, particularly for valves on natural gas pipelines. There has long been a need for a device and a method to power pipeline valve actuators, especially in an emergency, without discharging natural gas into the atmosphere or other injurious material, such as hydraulic fluid onto the ground or into the atmosphere.